Polyimide film and use thereof

ABSTRACT

A polyimide film in which the dimensional change is reduced when it has undergone a step of laminating a metal on the polyimide film or a step of etching the metal layer to form wiring, and the rate of dimensional change can be stabilized across the entire width is provided. The object can be solved by a polyimide film produced by a continuous process, wherein when a coefficient of linear expansion a in a direction of the molecular orientation axis and a coefficient of linear expansion b in a direction perpendicular to the molecular orientation axis are measured in the temperature range of 100° C. to 200° C., a and b satisfy a particular relationship across the entire width, or a polyimide film produced by a continuous process, wherein when a tear propagation resistance c in the direction of the molecular orientation axis and a tear propagation resistance d in the direction perpendicular to the molecular orientation axis are measured, c and d satisfy a particular relationship across the entire width.

RELATED APPLICATION

This application is a Continuation-in-Part of PCT application PCT/JP2005/004282 filed on Mar. 11, 2005, claiming priority based on Japanese Application No. 2004-0473558 and 2004-094901 filed on Mar. 15, 2004 and Mar. 29, 2004 respectively, the contents of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a polyimide film suitable for use in flexible printed circuit boards, tapes for tape automated bonding (TAB), substrates for electrical and electronic devices such as a substrate for a solar cell, high-density recording media, and magnetic recording media, and use thereof. More specifically, the present invention relates to a polyimide film in which the rate of dimensional change is reduced when it has undergone a step of forming a metal layer, in particular, a step of laminating a metal foil under heating or a step of etching the metal layer, and physical property values (rate of dimensional change) can be stabilized across the entire width of the film.

BACKGROUND ART

In the technical field of electronics, demand for high-density mounting has been increasing. Accordingly, in the technical field using a flexible printed circuit board (hereinafter referred to as “FPC”), physical properties etc. that are suitable for the high-density mounting have been desired.

A process of producing the FPC is broadly divided into (1) a step of laminating a metal onto a base film (hereinafter referred to as “metal lamination step”) and (2) a step of forming wiring with a desired pattern on the metal surface (hereinafter referred to as “wiring formation step”). In particular, in the process of producing the FPC for high-density mounting, a small dimensional change of the base film is desired.

In the metal lamination step and the wiring formation step, the dimensional change of the base film is particularly increased in the following stages. (1) In the metal lamination step, before and after a stage of laminating a metal while heating the base film. (2) In the wiring formation step, before and after etching for patterning the metal. Therefore, when the FPC for high-density mounting is produced, desirably, the dimensional change of the base film is small before and after these stages.

In the production of FPCS, a metal layer is laminated by a roll-to-roll processing of a base film with a large width. Therefore, it is desired that the physical properties of the base film be stable across the entire width (the entire width direction), that is, the rate of dimensional change be stable across the entire width of the base film.

A polyimide film containing a polyimide resin as a main component is suitably used as the base film. In the polyimide film used as the base film, various techniques have been proposed in order to control the rate of dimensional change.

For example, Patent Document 1 discloses a polyimide film produced by appropriately selecting monomer materials in which an average coefficient of linear expansion is about 1 to 25 ppm/° C. in the temperature range of about 50° C. to 300° C. and a coefficient of linear expansion ratio (MD/TD) in the machine direction (MD) and the transverse direction (TD) of the polyimide film is about ⅕ to 4 (see p. 1, claim 1; p. 2, lower left, lines 4 to 14; p. 3, lower right, lines 1 to 10; and the like of the document). In Patent Document 1, the dimensional stability of the film itself during heating is improved.

As a technique for controlling the dimensional changes by stretching a polyimide film in at least one direction, Patent Document 2 proposes a method of swelling a precursor film of polyimide containing a residual solvent with a swelling agent, and then stretching the film in at least a uniaxial direction (see claim 1, paragraphs [0007], [0028], and the like of the document). Patent Document 3 proposes a method of swelling a gel film of a polyimide-amic acid ester copolymer with a solvent, and then stretching the film under heating (see paragraphs [0035] to [0038], and the like of the document). Patent Document 4 proposes a method of biaxially stretching a gel film under a specified degree of swelling, thereby controlling the thermal coefficient of linear expansion in the in-plane direction to be 10 ppm/° C. or less (see claim 3, paragraphs [0015], [0023], [0042], [0045] and the like of the document).

Patent Document 5 proposes a method of stretching a resin film (polyimide film) used as a base film layer of a tape for TAB in a uniaxial direction at a ratio in a predetermined range on the basis of the glass transition temperature, and then annealing the film (see claims 4 and 5, paragraphs [0017], [0041], and the like of the document). Patent Document 6 proposes a method of stretching a polyimide film at a ratio of 1.0 to 1.5 in the MD direction and at a ratio of 0.5 to 0.99 in the TD direction in the production of the polyimide film see paragraphs [0021], [0044], and the like of the document). Patent Document 7 proposes a method of applying a zone stretching (a method of stretching in which molecular chains of a raw film are aligned so as to agglomerate in a reed blind shape) on a polyimide film at a temperature of 250° C. or higher (see p. 1, claim 1; p. 2, upper left, line 15 to upper right, line 6; p. 2, lower left, line 7 to p. 3, upper right line 4; and the like of the document).

Furthermore, as a technique for controlling the dimensional changes by specifying a condition during drying (during imidization) using a tenter furnace (a furnace in which both ends in the width direction of a film are fixed to perform heating) (for convenience, referred to as “tenter process technique”), Patent Document 8 proposes a method of sequentially decreasing the distance between the fixed ends of a film in the first half of a heating furnace, and sequentially increasing the distance in the second half of the heating furnace in producing a polyimide film by a tenter process (see paragraphs [0005], [0032], and the like of the document). Patent Document 9 proposes a method of producing a polyimide film wherein when a self-supporting film is being carried in a tenter furnace while both ends of the film are held, the width of the film between the gripping positions is gradually decreased to 0.95 times that of the gripping part during a step of increasing the temperature to 300° C. at which the shrinkage due to drying is almost completed. Thereby, a polyimide film having a coefficient of linear expansion (TD) of 17 to 24 ppm/° C. in the temperature range from 50° C. to 200° C. and a tensile modulus (TD) of 700 kgf/mm² or more is produced (see paragraphs [0020], [0021], and the like of the document).

However, none of Patent Documents 1 to 9 describes a film disclosed in the present invention in which the coefficient of linear expansion in a direction of the molecular orientation axis and the coefficient of linear expansion in a direction perpendicular to the molecular orientation axis (for convenience, this direction may be referred to as “a perpendicular direction”) satisfy a particular relationship. For example, in some cases, it is difficult to reduce the rate of dimensional change when a metal is continuously laminated on a film or when wiring is formed by etching the metal layer. Furthermore, the amounts of dimensional change are different between an end of the polyimide film and the central portion thereof. Consequently, it may be difficult to stabilize the rate of dimensional change across the entire width of the film.

On the other hand, Patent Documents 10 to 14 disclose polyimide films in which the composition, the thickness, the tensile modulus, and the tear propagation resistance are specified.

However, none of Patent Documents 10 to 14 describes a film disclosed in the present invention in which the tear propagation resistance c in a direction of the molecular orientation axis and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis satisfy a particular relationship. In addition, these polyimide films are aimed at improving the handleability during mounting on a substrate or the like and the punchability. Therefore, for example, when a metal is continuously laminated on a film or when wiring is formed by etching the metal layer, the generation of dimensional changes may not be satisfactorily suppressed. Furthermore, the amounts of dimensional change are different between an end portion of the polyimide film and the central portion thereof. Consequently, it may be difficult to stabilize the rate of dimensional change across the entire width of the film.

Polyimide films are generally produced by a tenter furnace process in which ends of the film are gripped with clips or pin seats, and the film is transferred through a high-temperature furnace to bake the film. However, when a polyimide film is produced by the tenter furnace process, the same phenomenon as that caused by anisotropy of molecular orientation (generally referred to as “bowing phenomenon”) described in, for example, Non-Patent Documents 1 and 2 occurs in the production process of the polyimide film. Consequently, the anisotropy of molecular orientation is generated at the ends of the film (in particular, a part located within about 50 cm from a film-gripping device). When such anisotropy is exhibited, for example, a difference in coefficient of linear expansion and a difference in dimensional change are generated in the width direction of the film.

The present inventors have found that when the ratio between the coefficient of linear expansion in a direction of the molecular orientation axis of a polyimide film and the coefficient of linear expansion in a direction perpendicular to the molecular orientation axis thereof satisfies a particular relationship, the rate of dimensional change in the case where a metal is continuously laminated on the film or wiring is formed by etching the metal layer is small, or the rate of dimensional change can be stabilized across the entire width of the film.

Furthermore, the present inventors have found that, in a polyimide film in which the ratio between the tear propagation resistance c in a direction of the molecular orientation axis of the polyimide film and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis thereof (for convenience, this direction may be referred to as “a perpendicular direction”) is within a specific range, the rate of dimensional change in the case where a metal is continuously laminated on the film or wiring is formed by etching the metal layer is small, and the rate of dimensional change can be stabilized across the entire width of the film.

-   Patent Document 1: Japanese Unexamined Patent Application     Publication No. 61-264028 (Publication Date: Nov. 21, 1986) -   Patent Document 2: Japanese Unexamined Patent Application     Publication No. 2002-1804 (Publication Date: Jan. 8, 2002) -   Patent Document 3: Japanese Unexamined Patent Application     Publication No. 2003-128811 (Publication Date: May 8, 2003) -   Patent Document 4: Japanese Unexamined Patent Application     Publication No. 2003-145561 (Publication Date: May 20, 2003) -   Patent Document 5: Japanese Unexamined Patent Application     Publication No. 8-174695 (Publication Date: Jul. 9, 1996) -   Patent Document 6: Japanese Unexamined Patent Application     Publication No. 11-156936 (Publication Date: Jun. 15, 1999) -   Patent Document 7: Japanese Unexamined Patent Application     Publication No. 63-197628 (Publication Date: Aug. 16, 1998) -   Patent Document 8: Japanese Unexamined Patent Application     Publication No. 2000-290401 (Publication Date: Oct. 17, 2000) -   Patent Document 9: Japanese Unexamined Patent Application     Publication No. 2002-179821 (Publication Date: Jun. 26, 2002) -   Patent Document 10: Japanese Unexamined Patent Application     Publication No. 11-246685 0009 -   Patent Document 11: Japanese Unexamined Patent Application     Publication No. 2000-244083 0010 and 0011 -   Patent Document 12: Japanese Unexamined Patent Application     Publication No. 2000-198969 0009 and 0010 -   Patent Document 13: Japanese Unexamined Patent Application     Publication No. 2000-208563 0008 and 0009 -   Patent Document 14: Japanese Unexamined Patent Application     Publication No. 2000-208564 0008 and 0009 -   Non-Patent Document 1: Kunisuke Sakamoto, Kobunshi Ronbunshu     (Japanese Journal of Polymer Science and Technology) Vol. 48, No.     11, pp. 671-678 (1991) -   Non-Patent Document 2: Chisato Nonomura et al., Journal     Seikei-Kakou, Vol. 4, No. 5, pp. 312 to 317 (1992)

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

In the known polyimide films, it may be difficult to reduce dimensional changes when the films have undergone a step of laminating a metal thereon or a step of forming wiring by etching the metal layer, and to stabilize the rate of dimensional change across the entire width of the film. In addition, a polyimide film that undergoes a small dimensional change before and after a step of producing an FPC using the polyimide film as a base film, for example, a step of laminating a metal on the base film, or a step of forming wiring with a desired pattern on the surface of the metal, in particular, the polyimide film that has a stable rate of dimensional change across the entire width of the film when the polyimide film is produced by laminating the metal by a roll-to-roll processing of the base film with a large width has not yet been provided. In order to solve this problem, the present inventors have conducted intensive studies and have made the present invention.

Means for Solving the Problems

The present invention can solve the above problem by means of the following novel polyimide films and laminates including the same.

-   1) A polyimide film produced by a continuous process, wherein when a     coefficient of linear expansion a in a direction of the molecular     orientation axis and a coefficient of linear expansion b in a     direction perpendicular to the molecular orientation axis are     measured in the temperature range of 100° C. to 200° C., a     coefficient of linear expansion ratio A represented by equation (1):     A=1+{(b−a)/(b+a)}×2  (1)     is in the range of 1.01 to 3.00 across the entire width. -   2) The polyimide film according to 1) above, wherein the difference     between the maximum A_(MAX) and the minimum A_(MIN) of the     coefficient of linear expansion ratio A is 0.30 or less. -   3) The polyimide film according to 1) or 2) above, wherein the     difference between the maximum and the minimum of the molecular     orientation angle of the polyimide is 40° or less across the entire     width. -   4) The polyimide film according to any one of 1) to 3) above,     wherein when the transferring direction (MD direction) during the     continuous production is defined as 0°, the molecular orientation     angle is within 0±20°. -   5) The polyimide film according to any one of 1) to 4) above,     wherein the average coefficient of linear expansion in the     temperature range of 100° C. to 200° C. is in the range of 5.0 to     25.0 ppm/° C. -   6) A laminate including the polyimide film according to any one     of 1) to 5) above. -   7) The laminate according to 6) above, further including at least a     metal layer. -   8) A flexible printed circuit board including the polyimide film     according to any one of 1) to 5) above as a base film. -   9) A polyimide film produced by a continuous process, wherein when a     tear propagation resistance c in a direction of the molecular     orientation axis and a tear propagation resistance d in a direction     perpendicular to the molecular orientation axis are measured, the     tear propagation resistance ratio d/c is in the range of 1.01 to     1.20 and the difference between the maximum and the minimum of the     tear propagation resistance ratio d/c is 0.10 or less across the     entire width. -   10) The polyimide film according to 9) above, wherein the difference     between the maximum and the minimum of the molecular orientation     angle of the polyimide film is 40° or less across the entire width. -   11) The polyimide film according to 9) or 10) above, wherein when     the transferring direction (MD direction) during the continuous     production is defined as 0°, the molecular orientation angle of the     polyimide film is within 0±20° across the entire width. -   12) A laminate including the polyimide film according to any one     of 9) to 11) above. -   13) The laminate according to 12) above, further including at least     a metal layer. -   14) A flexible printed circuit board including the polyimide film     according to any one of 9) to 11) above as a base film.

EFFECT OF THE INVENTION

A polyimide film of the present invention is a polyimide film produced by a continuous process wherein when the coefficient of linear expansion a in a direction of the molecular orientation axis and the coefficient of linear expansion b in a direction perpendicular to the molecular orientation axis are measured in the temperature range of 100° C. to 200° C., these coefficients of linear expansion satisfy a particular relationship across the entire width.

A polyimide film of the present invention is a polyimide film wherein when the tear propagation resistance c in a direction of the molecular orientation axis and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis are measured, a tear propagation resistance ratio d/c is in the range of 1.01 to 1.20.

According to these films, for example, when the polyimide film is used as a base film of an FPC, the rate of dimensional change generated before and after the lamination of a metal layer and an etching process thereof can be reduced, and the rate of dimensional change can be stabilized across the entire width. Consequently, for example, these films provide an advantage that a high-quality FPC permitting high-density mounting can be provided.

BEST MODE FOR CARRYING OUT THE INVENTION

In embodiments, the present invention will be described in detail, in the order of the outline of a polyimide film of the present invention, a representative example of a method of producing the polyimide film, and the use of the polyimide film.

(I) Polyimide Film of the Present Invention

The polyimide film of the present invention is suitably used as a base film of an FPC and has a reduced rate of dimensional change before and after the lamination of a metal layer and the etching process. In particular, in the polyimide film, the rate of dimensional change is stabilized across the entire width.

In general, in producing FPCs, rates of dimensional change are measured in advance before and after the etching process of a metal-clad laminate (hereinafter referred to as “CCL”) prepared by laminating a metal and before and after heat treatment, and a correction coefficient is determined on the basis of the values.

When the rate of dimensional change of the CCL is stabilized across the entire width, the amount of dimensional change can be estimated using a single correction coefficient across the entire width of the CCL. Therefore, in the above-described process for producing an FPC, the amount of dimensional change after a step of heating and the amount of dimensional change after etching can be estimated in advance. Consequently, for example, when metal wiring is formed on a metal layer of the CCL, the pattern wiring can be easily formed. Accordingly, the production yield is improved and the reliability of the connection of the pattern wiring can also be improved, which can markedly contribute to the improvement in, for example, the quality and the yield of the FPC.

On the other hand, when the rate of dimensional change varies depending on the positions of the film, it is difficult to estimate the amount of dimensional change using a single correction coefficient to produce the FPC. Therefore, only a part having a stable rate of dimensional change of the CCL must be selected for use, or a part having a stable physical properties of the polyimide film that particularly affect the rate of dimensional change of the CCL must be selected for use. Since the disposal part increases, this method has a low production yield.

In order to control the rate of dimensional change and the variation thereof across the entire width to low values even after the above process, at least a coefficient of linear expansion ratio A represented by equation (1): A=1+{(b−a)/(b+a)}×2  (1) is specified within a predetermined range across the entire width of the polyimide film. Preferably, the polyimide film satisfies a condition that the upper limit of the difference between the maximum and the minimum of the coefficient of linear expansion ratio A is specified. More preferably, the polyimide film satisfies a condition that the molecular orientation angle across the entire width of the polyimide film is specified.

According to another embodiment of the present invention, the polyimide film satisfies the conditions that when the tear propagation resistance c in a direction of the molecular orientation axis and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis are measured, a tear propagation resistance ratio d/c is specified within a predetermined range, and that the upper limit of the difference between the maximum and the minimum of the tear propagation resistance ratio is specified, and preferably satisfies the condition that the molecular orientation angle across the entire width of the polyimide film is specified.

The polyimide film thus obtained can exhibit excellent dimensional stability (refer to examples, in particular, the rate of dimensional change described below), and can be suitably used as a base film of an FPC or the like.

Furthermore, in the polyimide film of the present invention, it is also possible to control the rate of dimensional change to be a low value before and after compression bonding performed by a hot roll lamination method in which the polyimide film and a metal foil are continuously bonded by thermocompression bonding with a bonding material therebetween. When the metal foil is bonded by the hot roll lamination method, the material is often brought in a heating environment under applied tension. Therefore, this may cause the problem of the rate of dimensional change. However, the use of the specific polyimide film of the present invention can reduce the rate of dimensional change and can stabilize the rate of dimensional change across the entire width.

These conditions will now be specifically described.

<Coefficient of Linear Expansion Ratio A and Difference A_(DIF) in Coefficient of Linear Expansion Ratio>

The polyimide film of the present invention is continuously produced. In this case, a coefficient of linear expansion ratio A represented by equation (1): A=1+{(b−a)/(b+a)}×2  (1) satisfies 1.01≦A≦3.00 across the entire width of the polyimide film. Furthermore, the difference between the maximum of the coefficient of linear expansion ratio A and the minimum thereof (difference A_(DIF) in coefficient of linear expansion ratio) preferably satisfies A_(DIF)≦0.30.

In the polyimide film of the present invention, the term “entire width” means the entire part in the direction (width direction, TD direction) perpendicular to the transferring direction (MD direction) in which the film is continuously produced. The specific methods of measuring the physical property values (coefficient of linear expansion etc.) across the entire width are not particularly limited. However, as described in examples below, the physical property values are measured at three points of both end portions and the central portion along the TD direction of the polyimide film, and these measured values may be compared or utilized.

In general, when a film (polyimide film) is produced by a tenter furnace process, since a stress due to the shrinkage of the film concentrates on the ends of the film, the physical property values at the ends may be significantly different from that at the central portion. Therefore, it is reasonable to consider that the physical property value of the entire width of the film is expressed by measuring the respective physical property values at both end portions and the central portion.

In the polyimide film produced by a continuous process in the present invention, when the polyimide film has a length of 200 mm or more in the width direction, and a length of 20 m or more in the longitudinal direction, the advantages of the invention can be markedly achieved. The polyimide film produced by a continuous process in the present invention includes a film that is produced by slitting a film having the above width into certain dimensions in the width direction and the longitudinal direction of the film.

The molecular orientation axis represents the direction of the highest degree of molecular orientation in the X-Y plane of the polyimide film wherein the MD direction of the polyimide film is defined as the X-axis and the TD direction thereof is defined as the Y-axis. Any general-purpose measuring device may be used for measuring the molecular orientation axis and the device is not particularly limited. For example, in the present invention, as described in examples below, the measurement can be performed with a molecular orientation analyzer, e.g., trade name: MOA 2012A or trade name: MOA 6015, manufactured by Oji Scientific Instruments.

In order to measure the coefficient of linear expansion a in a direction of the molecular orientation axis and the coefficient of linear expansion b in a direction perpendicular to the molecular orientation axis of the polyimide film in the present invention, first, the molecular orientation axis is determined with the device. In the measurement of the molecular orientation axis, measurement samples (40 mm×40 mm) are prepared from both end portions and the central portion of the width direction of the polyimide film, and the molecular orientation axes of the measurement samples are measured. When the width of the sample is small, preferably, respective samples are obtained so that the sampling position is shifted in the MD direction. For example, when the film width is 100 mm, sampling is preferably performed so that the sampling position is shifted in the MD direction as shown in FIG. 1.

In the present invention, the coefficients of linear expansion of the polyimide film are measured with respect to both the direction of the molecular orientation axis and the direction perpendicular to the molecular orientation axis (referred to as “a perpendicular direction” for convenience). A specific measuring method will be described. The angle of the molecular orientation axis is measured as described above, and measurement samples are prepared on the basis of the molecular orientation axis. For example, as shown in FIG. 2, reed-shape measurement samples 3 and 4 of a polyimide film 2 are respectively cut out in a direction of the molecular orientation axis (D_(AL) in the figure) and the perpendicular direction (D_(VE)). The size of the measurement samples 3 and 4 is not particularly limited as long as the measurement samples 3 and 4 have a size suitable for the measurement of the coefficient of linear expansion.

The thermal expansion coefficients of the measurement samples 3 and 4 thus prepared are measured with a known measurement device. For example, the coefficient of linear expansion is measured with a thermomechanical analyzer (manufactured by Seiko Instruments Inc., trade name: TMA120C) as follows. The sample is first heated from room temperature to 400° C. under a nitrogen stream at a heating rate of 10° C./min. Subsequently, the sample is gradually cooled to room temperature and is heated again from room temperature to 400° C. The thermal expansion coefficient calculated in the temperature range of 100° C. to 200° C. is defined as the coefficient of linear expansion.

In the present invention, the coefficient of linear expansion ratio A of the polyimide film 2 is calculated from the measured coefficients of linear expansion using the following equation (1). In equation (1), the coefficient of linear expansion in a direction of the molecular orientation axis is defined as a, and the coefficient of linear expansion in the perpendicular direction is defined as b. A=1+{(b−a)/(b+a)}×2  (1)

The coefficient of linear expansion ratio A calculated using equation (1) is preferably in the range of 1.01 to 3.00 (1.01≦A≦3.00), and more preferably in the range of 1.01 to 2.00 (1.01≦A≦2.00). When the coefficient of linear expansion ratio of the polyimide film is within this range, the rate of dimensional change across the entire width can be reduced, and the dimensional stability across the entire width can be satisfactory, in particular, before and after the etching stage in the production of FPCs.

Furthermore, in the present invention, the difference between the maximum (A_(MAX)) and the minimum (A_(MIN)) of the coefficient of linear expansion ratio A across the entire width (difference A_(DIF) in coefficient of linear expansion ratio=A_(MAX)−A_(MIN)) is preferably 0.30 or less (A_(DIF)≦0.30), more preferably 0.25 or less (A_(DIF)≦0.25), and particularly preferably 0.20 or less (A_(DIF)≦0.20). A small difference A_(DIF) in coefficient of linear expansion ratio does not cause a problem. On the other hand, in order to realize satisfactory dimensional stability before and after the etching stage in the production of FPCs, the upper limit of the difference A_(DIF) in coefficient of linear expansion ratio is 0.30 or less. Thus, by specifying the difference A_(DIF) in coefficient of linear expansion ratio, the rate of dimensional change can be satisfactorily stabilized across the entire width of the polyimide film.

In the present invention, the molecular orientation axis and the coefficient of linear expansion are measured as described above. The relationship between the above method of preparing samples and the measured values is as follows. Namely, square measurement samples with dimensions of 4×4 cm are prepared from both end portions and the central portion of the polyimide film, and the molecular orientation axis is measured. Since the molecular orientation axis is determined, strip measurement samples are prepared in the direction of the molecular orientation axis and the direction perpendicular to the molecular orientation axis, and the coefficients of linear expansion of the samples are then measured. The coefficient of linear expansion ratio A and the difference A_(DIF) in coefficient of linear expansion ratio are calculated from the coefficients of linear expansion measured as described above.

<Average Coefficient of Linear Expansion C_(LE)>

In the polyimide film of the present invention, in addition to the coefficient of linear expansion ratio A and the difference A_(DIF) in coefficient of linear expansion ratio, an average coefficient of linear expansion C_(LE) is also preferably specified. The coefficient of linear expansion a in a direction of the molecular orientation axis and the coefficient of linear expansion b in the perpendicular direction are measured across the entire width, and the average coefficient of linear expansion C_(LE) is then calculated using the following equation (2). In equation (2), as shown in FIG. 2, the coefficient of linear expansion in the direction of the molecular orientation axis (D_(AL) in the figure) at an end is defined as C1 _(AL), the coefficient of linear expansion in the perpendicular direction (D_(VE)) at the end is defined as C1 _(VE), the coefficient of linear expansion in the direction of the molecular orientation axis (D_(AL) in the figure) at the center is defined as C0 _(AL), the coefficient of linear expansion in the perpendicular direction (D_(VE)) at the center is defined as C0 _(VE), the coefficient of linear expansion in the direction of the molecular orientation axis (D_(AL) in the figure) at another end is defined as C2 _(AL), and the coefficient of linear expansion in the perpendicular direction (D_(VE)) at the other end is defined as C2 _(VE). C _(LE)={(C1_(AL) +C1_(VE))/2+(C0_(AL) +C0_(VE))/2+(C2_(AL) +C2_(VE))/2}/3  (2)

In particular, when the polyimide film is used as a base film of an FPC, the average coefficient of linear expansion C_(LE) is preferably a value close to the average coefficient of linear expansion (16.3 ppm/° C.) of a metal layer (in particular, a copper layer) in the temperature range of 100° C. to 200° C., or lower than the value. The reason for this is as follows. From the viewpoint that the polyimide film is stretched during heat treatment in the production of FPC, when the average coefficient of linear expansion of the polyimide film is close to the coefficient of linear expansion of the copper layer, the dimensional stability can be naturally improved.

Accordingly, in the polyimide film of the present invention, the average coefficient of linear expansion in the temperature range of 100° C. to 200° C. is preferably in the range of 5.0 to 25.0 ppm/° C., more preferably in the range of 5.5 to 23.0 ppm/° C., and further preferably in the range of 6.0 to 20.0 ppm/° C. When the average coefficient of linear expansion of the polyimide film is within the above range, in the case where an FPC is produced using the polyimide film as a base film, the rate of dimensional change of the base film before and after etching can be reduced.

<Molecular Orientation Angle and Difference in Molecular Orientation Angle>

In the polyimide film of the present invention, in addition to the coefficient of linear expansion ratio A and the difference A_(DIF) in coefficient of linear expansion ratio, a molecular orientation angle across the entire width of the polyimide film is also preferably specified. With respect to the specification of the molecular orientation angle, at least one of the difference between the maximum and the minimum of the molecular orientation angle across the entire width of the polyimide film and a small variation in the molecular orientation angle across the entire width of the polyimide film is specified. This molecular orientation angle means the angle formed by the molecular orientation axis described in the above section <coefficient of linear expansion ratio A and difference A_(DIF) in coefficient of linear expansion ratio>.

First, regarding the difference between the maximum and the minimum of the molecular orientation angle (for convenience, referred to as “difference in molecular orientation angle”), the upper limit of the difference in molecular orientation angle is 40° or less and more preferably 30° or less. In the case where the upper limit of the difference in molecular orientation angle is within the above range, when an FPC is formed with a metal layer, the variation in the rate of dimensional change across the entire width before and after etching can be reduced, thus being preferable. In this case, the direction of the molecular orientation angle is not particularly limited, and may be any direction.

The molecular orientation angle in the present invention means the angle formed between the measured molecular orientation axis and the MD direction. The phrase “the molecular orientation angle of the polyimide film is 0°” means that the direction of the molecular orientation axis is a direction (the same direction as shown by 10 in FIG. 3) parallel to the MD direction (the same direction as shown by 14 in FIG. 3). The term “positive (plus) molecular orientation angle” represents the case where the angle is tilted from the MD direction in the counterclockwise direction (11 in FIG. 3). On the other hand, the term “negative (minus) molecular orientation angle” represents the case where the angle is tilted from the MD direction in the clockwise direction (12 in FIG. 3). The difference in molecular orientation angle in the present invention can be measured as follows. First, the molecular orientation angles are measured across the width direction of the film. The difference in molecular orientation angle can be calculated using the following equation (3) from the maximum positive molecular orientation angle and the maximum negative molecular orientation angle. When only positive molecular orientation angles are observed across the width direction, equation (4) is used. When only negative molecular orientation angles are observed across the width direction, equation (5) is used. When the maximum or the minimum of the molecular orientation angle is 0°, the difference in molecular orientation angle can be determined as follows. When 0° is the maximum, the difference in molecular orientation angle is determined by equation (6) using the minimum negative molecular orientation angle. When 0° is the minimum, the difference in molecular orientation angle is determined by equation (7) using the maximum positive molecular orientation angle. Difference in molecular orientation angle=(positive molecular orientation angle)−(negative molecular orientation angle)  (3) Difference in molecular orientation angle=(the maximum positive molecular orientation angle)−(the minimum positive molecular orientation angle)  (4) Difference in molecular orientation angle=(the minimum negative molecular orientation angle)−(the maximum negative molecular orientation angle)  (5) Difference in molecular orientation angle=0−(the minimum negative molecular orientation angle)  (6) Difference in molecular orientation angle=(the maximum positive molecular orientation angle)  (7) The difference between the maximum and the minimum of the molecular orientation angle in the present invention means a value calculated from any of the above equations using values among the molecular orientation angles at both end portions of a polyimide film and the molecular orientation angle at the central portion thereof.

The direction of the molecular orientation angle may be any direction as long as the difference in the molecular orientation angle is 40° or less. Preferably, the difference in the molecular orientation angle is 30° or less. When the difference between the maximum and the minimum of the molecular orientation angle is 40° or less, the variation in the amount of dimensional change can be reduced across the entire width of the film, thus being preferable.

Furthermore, in the present invention, the variation in the molecular orientation angle is specified. Namely, the molecular orientation angle is specified so as to be within 0±20° across the entire width of the polyimide film. When the film-transferring direction (MD direction) of the polyimide film is defined as a reference (0°) (10 in FIG. 3), the molecular orientation angle of the polyimide film is preferably within 0±20° across the entire width. The state that the molecular orientation angle in the present invention is within 0±20° can be described with reference to a drawing, i.e., FIG. 3, showing the relationship between the film-transferring direction (14 in FIG. 3) and the molecular orientation angle. The molecular orientation angle of 0° of the polyimide film means that the direction of the molecular orientation axis is parallel (10 in FIG. 3) to the MD direction. The molecular orientation angle of 20° represents the case where the angle is tilted in the counterclockwise direction from the MD direction (the angle shown by 11 in FIG. 3 being 20°). On the other hand, the molecular orientation angle of −20° represents the case where the angle is tilted in the clockwise direction from the MD direction (the angle shown by 12 in FIG. 3 being −20°). That is, the molecular orientation angle of 0±20°, which is preferred in the present invention, means that the molecular orientation angle is controlled so as to be within 20° in the left and right directions with respect to the MD direction.

An example of a method of producing a metal-clad laminate using a polyimide film as a base film is a method of applying an adhesive on the polyimide film, and then performing a thermocompression bonding with a metal foil (hot lamination method). In this method, during thermocompression bonding, the polyimide film is stretched with a thermocompression bonding apparatus in the MD direction of the film and is shrunk in the TD direction. When the molecular orientation axis is controlled to within 0±20°, the polyimide film is uniformly stretched in the MD direction across the entire width of the film. For example, in the case of a film having a width of 250 mm or more, uniform tension is applied across the entire width of the polyimide film along the MD direction. Consequently, when the polyimide film is stretched under heating, a stretching at one side of the film and curling of the film, which are caused by a difference in the elongation ratio between both end portions of the film, can also be suppressed. Accordingly, by controlling the molecular orientation angle as descried above, in particular, when the hot lamination method is used, the rate of dimensional change can be decreased, and the rate of dimensional change can be stabilized across the entire width.

<Tear Propagation Resistance c in a Direction of Molecular Orientation Axis, Tear Propagation Resistance d in a Direction Perpendicular to Molecular Orientation Axis, and Tear Propagation Resistance Ratio d/c>

According to another embodiment of the polyimide film of the present invention, in a polyimide film produced by a continuous process, when the tear propagation resistance c in a direction of the molecular orientation axis and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis are measured, the tear propagation resistance ratio d/c is in the range of 1.01 to 1.20, and more preferably in the range of 1.01 to 1.15 across the entire width of the polyimide film.

The tear propagation resistance c in a direction of the molecular orientation axis and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis of the polyimide film in the present invention can be measured by the same procedures as those of the coefficient of linear expansion ratio A and the difference A_(DIF) in coefficient of linear expansion ratio. Namely, measurement samples (40 mm×40 mm) are prepared from both end portions and the central portion of the width direction of the polyimide film, and the molecular orientation axes of the measurement samples are measured to determine the molecular orientation axes. Subsequently, as shown in FIG. 4, each measurement sample is respectively cut out (21 and 22 in FIG. 4) in a direction of the molecular orientation axis (20 in FIG. 4) and in a direction perpendicular to the molecular orientation axis (23 in FIG. 4). The tear propagation resistances of the cut out test pieces (10 mm×20 mm) are then measured to determine the above values. The tear propagation resistance is measured in accordance with American Society for Testing and Materials (ASTM) D1938 using the cut out test piece.

In the present invention, in order to reduce the rate of dimensional change, the tear propagation resistance ratio calculated with the following equation (8) using the tear propagation resistance c in a direction of the molecular orientation axis and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis is preferably in the range of 1.01 to 1.20, and more preferably in the range of 1.01 to 1.15. Tear propagation resistance ratio=d/c  (8)

By controlling the tear propagation resistance ratio of the polyimide film to be within the above range, the rate of dimensional change of the polyimide film can be suppressed to a low value and the rate of dimensional change can be stabilized across the entire width of the film, thus being preferable.

Furthermore, in the present invention, from the viewpoint that not only the rate of dimensional change but also the variation in the physical property value (rate of dimensional change) in the width direction of the film can be reduced, the difference between the maximum and the minimum of the tear propagation resistance ratio is preferably 0.10 or less. In the phrase “the difference between the maximum and the minimum in the present invention is 0.10 or less”, the difference means a value obtained by calculating the difference between the maximum and the minimum among the tear propagation resistance ratios at both end portions of the polyimide film and the tear propagation resistance ratio at the central portion thereof by the following equation (9). Difference between the maximum and the minimum of tear propagation resistance ratio=the maximum tear propagation resistance ratio−the minimum tear propagation resistance ratio  (9)

First, regarding the difference between the maximum and the minimum of the molecular orientation angle (for convenience, referred to as “difference in molecular orientation angle”), the upper limit of the difference in molecular orientation angle is 40° or less and more preferably 30° or less. In the case where the upper limit of the difference in molecular orientation angle is within the above range, when an FPC is formed with a metal layer, the variation in the rate of dimensional change in a base film including the polyimide can be reduced across the entire width, thus being preferable. In this case, the direction of the molecular orientation angle is not particularly limited, and may be any direction.

(II) Method of Producing a Polyimide Film of the Present Invention

The method of producing a polyimide film of the present invention is not particularly limited. The type of the polyimide resin is also not particularly limited. However, a method of changing the production conditions of the film can be used as means for obtaining a polyimide film wherein the coefficient of linear expansion ratio A represented by equation (1) is in the range of 1.01 to 3.00 across the entire width of the film, or a polyimide film of another embodiment of the present invention wherein when the tear propagation resistance c in a direction of the molecular orientation axis and the tear propagation resistance d in a direction perpendicular to the molecular orientation axis are measured, the tear propagation resistance ratio d/c is in the range of 1.01 to 1.20. In order to obtain a desired polyimide film, for example, a production method including

-   (A) a step of preparing a polyamic acid by polymerization, -   (B) a step of casting or applying a composition containing the     polyamic acid and an organic solvent on a support, and then forming     a gel film, -   (C) a step of peeling off the gel film and fixing both ends of the     gel film, and -   (D) a step of transferring the film in a heating furnace with both     ends of the film being fixed can be employed. The above polyimide     film can be produced by appropriately selecting each of these     conditions or adding other steps. Examples of the variable     production conditions and production examples will be described     below.

Step (A)

Step (A) is a process for polymerizing a polyamic acid. The polyamic acid is not particularly limited, but a polyamic acid solution prepared by allowing an acid dianhydride to react with a substantially equimolar amount of diamine in an organic solvent may be used.

Any known polymerization method can be used. Examples of particularly preferable polymerization method include the following methods:

-   1) A polymerization method in which an aromatic diamine is dissolved     in a polar organic solvent, and is then allowed to react with a     substantially equimolar amount of an aromatic tetracarboxylic     dianhydride. -   2) A polymerization method in which an aromatic tetracarboxylic     dianhydride is allowed to react with fewer moles of an aromatic     diamine compound in a polar organic solvent to prepare a prepolymer     having acid anhydride groups at both terminals, and the aromatic     diamine compound is then added so that the amount of the aromatic     tetracarboxylic dianhydride and that of the aromatic diamine     compound are substantially equimolar in the whole process. -   3) A polymerization method in which an aromatic tetracarboxylic     dianhydride is allowed to react with excessive moles of an aromatic     diamine compound in a polar organic solvent to prepare a prepolymer     having amino groups at both terminals, the aromatic diamine compound     is further added to the prepolymer, and the aromatic tetracarboxylic     dianhydride is then added so that the amount of the aromatic     tetracarboxylic dianhydride and that of the aromatic diamine     compound are substantially equimolar in the whole process. -   4) A polymerization method in which an aromatic tetracarboxylic     dianhydride is dissolved and/or dispersed in a polar organic solvent     and substantially equimolar amount of an aromatic diamine compound     is then added. -   5) A polymerization method in which a mixture of substantially     equimolar amounts of an aromatic tetracarboxylic dianhydride and an     aromatic diamine is allowed to react in a polar organic solvent.

Examples of the organic solvent used in the preparation of the polyamic acid by polymerization include aprotic solvents such as ureas, e.g., tetramethylurea and N,N-dimethylethylurea; sulfoxides or sulfones, e.g., dimethyl sulfoxide, diphenylsulfone, and tetramethylsulfone; amides, e.g., N,N-dimethylacetamide (abbreviated as DMAc), N,N-dimethylformamide (abbreviated as DMF), N-methyl-2-pyrrolidone (abbreviated as NMP), γ-butyrolactone, and hexamethylphosphoric triamide; aprotic solvents, e.g., phosphorylamides; alkyl halides such as chloroform and methylene chloride; aromatic hydrocarbons such as benzene and toluene; phenols such as phenol and cresol; and ethers such as dimethyl ether, diethyl ether, and p-cresol methyl ether. These solvents are generally used alone, but two or more solvents may be used in appropriate combinations properly. Among them, amides such as DMF, DMAc, and NMP are preferably used as the solvent in view of high solubility of polymers.

Regarding the weight percent of the solid content of the polyamic acid in the polyamic acid solution, in view of handling, 5 to 40 weight percent, preferably 10 to 30 weight percent, and more preferably 13 to 25 weight percent of the polyamic acid is dissolved in an organic solvent. In view of the physical properties of the film, the average molecular weight of the polyamic acid is 10,000 or more in terms of polyethylene glycol (PEG) by gel permeation chromatography (GPC).

The viscosity of the polyamic acid solution is measured with a B-type viscometer including a rotor No. 7 at a rotational speed of 4 rpm after the polyamic acid solution is kept for one hour in a water bath kept at 23° C. In view of easy handling in the production of a formed film, the viscosity is preferably in the range of 50 to 1,000 Pa·s, more preferably 100 to 500 Pa·s, and most preferably 200 to 350 Pa·s.

Examples of the acid dianhydride suitably used in the production of the polyamic acid solution of the present invention include acid ester anhydrides such as p-phenylene bis(trimellitic acid monoester anhydride), p-methylphenylene bis(trimellitic acid monoester anhydride), p-(2,3-dimethylphenylene) bis(trimellitic acid monoester anhydride), 4,4′-biphenylene bis(trimellitic acid monoester anhydride), 1,4-naphthalene bis(trimellitic acid monoester anhydride), 2,6-naphthalene bis(trimellitic acid monoester anhydride), and 2,2-bis(4-hydroxyphenyl)propanedibenzoate-3,3′,4,4′-tetracarboxylic dianhydride; and dianhydrides such as ethylenetetracarboxylic dianhydride, 1,2,3,4-butanetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, pyromellitic dianhydride, 1,2,3,4-benzenetetracarboxylic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, 2,2′,3,3′-biphenyltetracarboxylic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, 2,2′,3,3′-benzophenonetetracarboxylic dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)sulfone dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,3,6,7-anthracenetetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 4,4-(p-phenylenedioxy)diphthalic dianhydride, 4,4-(m-phenylenedioxy)diphthalic dianhydride, and 2,2-bis[(2,3-dicarboxyphenoxy)phenyl]propane dianhydride. These may be used alone or in combinations of two or more.

Among them, at least one acid dianhydride selected from pyromellitic dianhydride, 1,2,3,4-benzenetetracarboxylic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, 2,2′,3,3′-biphenyltetracarboxylic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, 2,2′,3,3′-benzophenonetetracarboxylic dianhydride, and p-phenylenebis(trimellitic acid monoester anhydride) is preferably used because heat resistance is imparted to the polyimide film, and the elastic modulus of the film is improved, thereby easily controlling the orientation angle of the polyimide film.

Furthermore, the use of the above acid dianhydride is preferred because the average coefficient of linear expansion can be controlled to a preferable range.

When the elastic modulus of the polyimide film is improved, a volumetric shrinkage during volatilization of residual volatile components in the film generates a shrinkage stress in the plane of the film. The molecular orientation in the plane is accelerated by the shrinkage stress. As a result, the molecular orientation of the polyimide film proceeds. In addition, when the in-plane orientation proceeds, the average coefficient of linear expansion of the polyimide film can be controlled to be a low value. Thus, the average coefficient of linear expansion can be controlled to the preferable range in the present invention.

Examples of the amine compounds include p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, 3,3′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl ether, 3,3′-diaminodiphenyl sulfide, 3,4′-diaminodiphenyl sulfide, 4,4′-diaminodiphenyl sulfide, 3,3′-diaminodiphenyl sulfone, 3,4′-diaminodiphenyl sulfone, 4,4′-diaminodiphenyl sulfone, 3,3′-diaminobenzophenone, 3,4′-diaminobenzophenone, 4,4′-diaminobenzophenone, 3,3′-diaminodiphenylmethane, 3,4′-diaminodiphenylmethane, 4,4′-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 2,2-bis(3-aminophenyl)propane, 2-(3-aminophenyl)-2-(4-aminophenyl)propene, 2,2-bis(4-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 2,2-bis(3-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 2-(3-aminophenyl)-2-(4-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminobenzoyl)benzene, 1,4-bis(3-aminobenzoyl)benzene, 1,3-bis(4-aminobenzoyl)benzene, 1,4-bis(4-aminobenzoyl)benzene, 3,3′-diamino-4-phenoxybenzophenone, 4,4′-diamino-5-phenoxybenzophenone, 3,4′-diamino-4-phenoxybenzophenone, 3,4′-diamino-5-phenoxybenzophenone, 4,4′-bis(4-aminophenoxy)biphenyl, 3,3′-bis(4-aminophenoxy)biphenyl, 3,4′-bis(3-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl]ketone, bis[4-(3-aminophenoxy)phenyl]ketone, bis[3-(4-aminophenoxy)phenyl]ketone, bis[3-(3-aminophenoxy)phenyl]ketone, 3,3′-diamino-4,4′-diphenoxydibenzophenone, 4,4′-diamino-5,5′-diphenoxydibenzophenone, 3,4′-diamino-4,5′-diphenoxydibenzophenone, bis[4-(4-aminophenoxy)phenyl]sulfide, bis[3-(4-aminophenoxy)phenyl]sulfide, bis[4-(3-aminophenoxy)phenyl]sulfide, bis[3-(4-aminophenoxy)phenyl]sulfide, bis[3-(3-aminophenoxy)phenyl]sulfide, bis[3-(4-aminophenoxy)phenyl]sulfone, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[3-(3-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]ether, bis[4-(4-aminophenoxy)phenyl]ether, bis[3-(3-aminophenoxy)phenyl]ether, bis[4-(3-aminophenoxy)phenyl]methane, bis[4-(4-aminophenoxy)phenyl]methane, bis[3-(3-aminophenoxy)phenyl]methane, bis[3-(4-aminophenoxy)phenyl]methane, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[3-(3-aminophenoxy)phenyl]propane, 2,2-bis[4-(3-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 2,2-bis[3-(3-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 2,2-bis[3-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 1,4-bis[4-(3-aminophenoxy)benzoyl]benzene, 1,3-bis[4-(3-aminophenoxy)benzoyl]benzene, 1,3-bis(3-amino-4-phenoxybenzoyl)benzene, 1,4-bis(3-amino-4-phenoxybenzoyl)benzene, 1,3-bis(4-amino-5-phenoxybenzoyl)benzene, 1,3-bis(4-amino-5-biphenoxybenzoyl)benzene, 1,4-bis(4-amino-5-biphenoxybenzoyl)benzene, 1,3-bis(3-amino-4-biphenoxybenzoyl)benzene, 1,4-bis(3-amino-4-biphenoxybenzoyl)benzene, 1,4-bis[4-(4-aminophenoxy)-α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-aminophenoxy) -α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-amino-6-trifluoromethylphenoxy)-α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-amino-6-fluoromethylphenoxy)-α,α-dimethylbenzyl]benzene, 1,3-bis(4-(4-amino-6-methylphenoxy)-α,α-dimethylbenzyl]benzene, 1,3-bis[4-(4-amino-6-cyanophenoxy)-α,α-dimethylbenzyl]benzene, and diaminopolysiloxane. These may be used alone or in combinations of two or more.

Among them, at least one compound selected from p-phenylenediamine, m-phenylenediamine, 3,3′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl ether, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane is preferably used from the standpoint that heat resistance of the polyimide film can be improved and the rigidity can be imparted to the film. Furthermore, p-phenylenediamine and/or 3,4′-diaminodiphenyl ether is preferably combined as an essential component because the elastic modulus of the polyimide film is improved, thereby easily controlling the orientation angle of the polyimide film. In addition, since the in-plane orientation proceeds, the average coefficient of linear expansion of the polyimide film can be controlled to be a low value. Thus, the average coefficient of linear expansion can be controlled to the preferable range in the present invention.

In the present invention, a high elastic modulus of the polyimide film is preferred from the viewpoint that the molecular orientation angle can be easily controlled to a preferable range. The elastic modulus was calculated as follows. The tensile moduli of the MD direction and the TD direction were measured using a sample prepared from the central portion (5 in FIG. 1) of the polyimide film. The elastic modulus was calculated using an equation: elastic modulus={tensile modulus in the MD direction+tensile modulus in the TD direction}/2. In view of the control of the orientation angle, the elastic modulus is preferably 4.2 GPa or more. The upper limit thereof is preferably 10.0 GPa or less because the flexibility during handling is not satisfactory.

Examples of particularly preferred polyimide film include:

1. A polyimide film prepared using four monomers including p-phenylenediamine, 4,4′-diaminodiphenyl ether, pyromellitic dianhydride, and p-phenylenebis(trimellitic acid monoester anhydride);

2. A polyimide film prepared using p-phenylenediamine, 4,4′-diaminodiphenyl ether, pyromellitic dianhydride, and 3,3′,4,4′-biphenyltetracarboxylic dianhydride;

3. A polyimide film prepared using p-phenylenediamine, 4,4′-diaminodiphenyl ether, pyromellitic dianhydride, and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride;

4. A polyimide film prepared using p-phenylenediamine, 4,4′-diaminodiphenyl ether, pyromellitic dianhydride, p-phenylenebis(trimellitic acid monoester anhydride), and 3,3′,4,4′-biphenyltetracarboxylic dianhydride;

5. A polyimide film prepared using p-phenylenediamine, 4,4′-diaminodiphenyl ether, and 3,3′,4,4′-biphenyltetracarboxylic dianhydride;

6. A polyimide film prepared using 4,4′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, and pyromellitic dianhydride;

7. A polyimide film prepared using p-phenylenediamine, 4,4′-diaminodiphenyl ether, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, pyromellitic dianhydride, and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride; and

8. A polyimide film prepared using p-phenylenediamine and 3,3′,4,4′-biphenyltetracarboxylic dianhydride. These polyimide films are advantageous in that the orientation of the film can be easily controlled and that the coefficient of linear expansion thereof can be controlled to be a low value, and thus are suitably used.

Step (B)

In step (B), a composition (also referred as a polyamic acid solution) containing the polyamic acid and an organic solvent is cast or applied on a support to form a gel film. The composition used in step (B) may contain another component such as a reagent that can react with the polyamic acid.

The viscosity of the polyamic acid solution is measured with a B-type viscometer with a rotor No. 7 at a rotational speed of 4 rpm after the polyamic acid solution is kept for one hour in a water bath kept at 23° C. In view of easy handling in the production of a formed film, the viscosity is preferably in the range of 50 to 1,000 Pa·s, more preferably 100 to 500 Pa·s, and most preferably 200 to 350 Pa·s.

The solid content of the polyamic acid in the polyamic acid solution used in step (B) is in the range of 5 to 40 weight percent, preferably 10 to 30 weight percent, and more preferably 13 to 25 weight percent. In this range, the polyamic acid solution tends to be easily handled in the production of a formed film.

The viscosity and concentration of the polyamic acid solution can be controlled by, according to need, adding an organic solvent such as the polymerization solvent for polyamic acid described in step (A).

The polyimide film can be produced from the polyamic acid solution by a known method. The method includes a thermal imidization method and a chemical imidization method. In the thermal imidization method, imidization is accelerated by only heating. The heating conditions can be changed according to the type of the polyamic acid used, the thickness of the film, and the like. Furthermore, imidization is preferably performed using a polyamic acid solution containing a releasing agent, a thermal imidization catalyst, and the like, according to need. In the chemical imidization method, the polyamic acid solution is treated with an imidization catalyst and a dehydrating agent. Examples of the dehydrating agent include aliphatic acid anhydrides such as acetic anhydride, and aromatic acid anhydrides such as benzoic anhydride. Examples of the imidization catalyst include aliphatic tertiary amines such as triethylamine; aromatic tertiary amines such as dimethylaniline; and heterocyclic tertiary amines such as pyridine, picoline, and isoquinoline.

The amount of the imidization catalyst used is not particularly limited. However, the molar ratio of imidization catalyst/amido group in polyamic acid is preferably in the range of 10 to 0.01 and more preferably in the range of 5 to 0.5.

When the dehydrating agent and the imidization catalyst are combined, preferably, the molar ratio of dehydrating agent/amido group in polyamic acid is in the range of 10 to 0.01, and the molar ratio of imidization catalyst/amido group in polyamic acid is in the range of 10 to 0.01. More preferably, the molar ratio of dehydrating agent/amido group in polyamic acid is in the range of 5 to 0.5, and the molar ratio of imidization catalyst/amido group in polyamic acid is in the range of 5 to 0.5. In this case, a reaction retardant such as acetylacetone may be combined. The ratios of the dehydrating agent and the imidization catalyst to the polyamic acid may be determined by the time (pot life) required from the mixing of the polyamic acid and the dehydrating agent/catalyst mixture at 0° C. to the start of an increase in the viscosity. The pot life is generally in the range of 0.1 to 120 minutes and preferably in the range of 1 to 60 minutes.

Additives such as a thermal stabilizer, an antioxidant, an ultraviolet absorber, an antistatic agent, a flame retardant, a pigment, a dye, a fatty acid ester, an organic lubricant (e.g. wax) may also be added. Furthermore, in order to impart lubricity, abrasion resistance, scratch resistance, and the like to the film surface, inorganic particles of clay, mica, titanium oxide, calcium carbonate, kaolin, talc, wet- or dry-process silica, colloidal silica, calcium phosphate, calcium hydrogen phosphate, barium sulfate, alumina, zirconia, or the like, or organic particles containing acrylic acids, styrene, or the like as the component may be added.

When the polyamic acid solution containing the imidization catalyst, the dehydrating agent, the additives, and the like is prepared, a step of removing insoluble raw materials and foreign substances through a filter or the like is preferably performed before mixing these components in order to reduce the foreign substances and defects in the film. The opening of the filter is ½, preferably ⅕, and more preferably 1/10 of the thickness of the film produced.

The polyamic acid solution prepared as described above is continuously cast or applied on a support, and is then dried to form a gel film. As the support, any support can be used as long as the support is not dissolved by the solution resin and can resist heating that is necessary for removing the organic solvent from the polyimide solution. In particular, an endless belt or a metal drum, which is produced by joining metal plates together, is preferred for drying the coated solution. The material of the endless belt or the metal drum is preferably a metal, and in particular, a SUS (stainless steel) material is preferably used. The surface of the support is preferably plated with a metal such as chromium, titanium, nickel, or cobalt because the adhesiveness of the solvent to the surface is improved or a dry organic insulating film can be easily peeled off. The endless belt or the metal drum preferably has a smooth surface. Alternatively, innumerable irregularities may be formed on the endless belt or the metal drum. The irregularities processed on the endless belt or the metal drum preferably have a diameter of 0.1 to 100 μm and a depth of 0.1 to 100 μm. By forming the irregularities on the metal surface, fine projections can be formed on the surface of the organic insulating film. Consequently, the projections can suppress the occurrence of flaws caused by friction between films or can improve slippage between the films.

The gel film of the present invention will be described. The term “gel film” means a state in which a part of the organic solvent or reaction products (referred to as “residual component”) remain in the film after the organic solvent solution containing the polyamic acid and the organic solvent is dried by heating. In the process of producing the polyimide film, the organic solvent dissolving the polyamic acid solution, the imidization catalyst, the dehydrating agent, and the reaction products (hygroscopic component of the dehydrating agent and water) remain as the residual component in the gel film. The ratio of the residual component in the gel film is calculated using the following equation (10): c=b/a×100  (10) wherein a represents the weight (g) of completely dry polyimide, b represents the weight (g) of the residual component contained in the gel film, and c represents the ratio of the residual component. The ratio of the residual component is preferably 500% or less, more preferably in the range of 25% to 200%, and particularly preferably in the range of 30% to 150%.

When the ratio of the residual component is more than 500%, handleability is not satisfactory, and the film is significantly shrinks during removal of the solvent. Consequently, undesirably, it is difficult to stabilize the orientation angle or the physical property values (rate of dimensional change) across the entire width of the film. The ratio of the residual component is preferably 25% or more because the orientation angle of the polyimide film is easily aligned in the MD direction (0°), and the physical property values of the film in the width direction is easily stabilized.

The weight a of completely dry polyimide and the weight b of the residual component are calculated as follows: The weight d of a gel film of 100 mm×100 mm is measured. The gel film is then dried in an oven at 300° C. for 20 minutes, and cooled to room temperature. The weight of the dry gel film is measured as the weight a of completely dry polyimide. The weight b of the residual component is calculated from the weight d of the gel film and the weight a of completely dry polyimide according to the equation of b=d−a.

In the step of producing the gel film, the temperature, wind speed, and exhaust speed during drying by heating on the support are preferably determined so that the ratio of the residual component is in the above range. In particular, in the production process of the polyimide film, the organic solvent solution containing the polymer and the organic solvent is preferably dried by heating at a temperature in the range of 50° C. to 200° C. and particularly preferably 50° C. to 180° C. The drying time is preferably in the range of 1 to 300 minutes, and the gel film is preferably dried under a multi-stage temperature control system.

Step (C)

In step (C), the gel film is peeled off from the support and the both ends of the gel film are continuously fixed. In the step of fixing the ends of the gel film in the present invention, ends of the gel film are gripped using gripping devices that are generally used in a film-producing apparatus, such as pin seats, clips, or the like. For example, the end gripping positions in the present invention are positions 37 shown in FIG. 5 at which gripping of the film ends with end gripping devices (pin seats or clips) is started in a film transfer apparatus 31 shown in FIG. 5.

As a method of fixing the gel film so that substantially no tension is applied in the TD direction during at least a part of step (D) described below, when the ends of the gel film are fixed in step (C), the gel film may be fixed so that substantially no tension is applied in the TD direction. That is, the film is fixed so that substantially no tension is applied in the TD direction, and is then transferred to step (D) as it is. Specifically, the ends of the film are fixed so that the film sags.

Step (D)

In step (D), the film is transferred in a heating furnace with both ends of the film being fixed. In the present invention, from the standpoint that the polyimide film intended in the present invention is produced, it is important that the film is transferred while being fixed so that substantially no tension is applied in the film width direction (TD direction) during at least a part (referred to as “step (D-1)”) of step (D).

The phrase “substantially no tension is applied in the TD direction” means that tensile tension due to mechanical handling is not applied in the TD direction except for the tension due to the weight of the film itself. Substantially, this means that the width (39 in FIG. 6) of the film between both fixed ends is larger than the distance (38 in FIG. 6) between both fixed ends of the film. The film under such a condition is referred to as “a film on which substantially no tension is applied”. This state will be described with reference to FIG. 6. The film is fixed by the gripping devices, and a length (38 shown in FIG. 6) in this state corresponds the distance between the ends of both end gripping devices. Tension is usually applied at both ends of the film, and, in this state, the distance 38 between both fixed ends is the same as the width 39 of the film between both fixed ends. In the present invention, as shown in FIG. 6, the distance 38 between both fixed ends is different from the width 39 of the film between both fixed ends, that is, the distance between both fixed ends is smaller than the width of the film. Specifically, the film is fixed so as to have a sag. In particular, from the standpoint that desirable characteristics of the present invention can easily exhibit, the film is preferably fixed so as to satisfy the following formula: 20.0>(Y−X)/Y×100>0.00  (11) wherein X represents the distance 38 between both fixed ends, and Y represents the width 39 of the film between both fixed ends. When the ratio (Y−X)/Y×100 (may be referred to as “TD shrinkage ratio” for convenience) exceeds the above range, it is difficult to stably control the sag of the film, and the amount of sag may vary in the transfer direction. Furthermore, in some cases, the film may drop from the end gripping devices because of the sag of the film, and thus the film cannot be stably produced. The ratio is more preferably 15.0≧(Y−X)/Y×100>0.00, and particularly preferably 10.0≧(Y−X)/Y×100>0.00.

In the present invention, the film is preferably fixed so that substantially no tension is applied in the TD direction at the entrance of the heating furnace in step (D) from the standpoint that the film can be controlled so as to have the most desirable orientation state across the entire width of the film, thereby producing the polyimide film that exhibits the desirable characteristics in the present invention. In order that the film is transferred while being fixed so that substantially no tension is applied in the TD direction at the entrance of the heating furnace, the following methods can be employed. In step (C), both ends of the gel film are fixed so that substantially no tension is applied in the TD direction, the film is then transferred to step (D) as it is (first method). An operation (process shown in FIG. 5) of decreasing the distance between both fixed ends is performed after step (C), and the film is then transferred to step (D) (second method). In the first method, both ends of the gel film are preferably fixed so as to satisfy formula (11). In the second method, the distance between the fixed ends is preferably decreased so as to satisfy formula (11).

After the first or second method is performed and the gel film is then introduced into the heating furnace in step (D), the operation of decreasing the distance between both fixed ends may be performed (third method). In the third method, the operation of decreasing the distance between both fixed ends is preferably performed in the temperature range of 300° C. or lower, more preferably 250° C. or lower, and particularly preferably 200° C. or lower. When the operation in the third method is performed in the temperature range over 300° C., it may be difficult to control the difference between the maximum A_(MAX) and the minimum A_(MIN) of the coefficient of linear expansion ratio A to a low value or control the difference between the maximum and the minimum of the tear propagation resistance ratio to a low value across the entire width of the film.

In the present invention, as described above, it is important that the gel film undergoes a state in which substantially no tension is applied in the TD direction immediately before a temperature is applied to the gel film.

In step (D), since the film is dried and imidization further proceeds, the film is shrunk to some extent. Therefore, when the film is transferred while being fixed so that substantially no tension is applied in the TD direction at the entrance of the heating furnace, the width of the film is decreased because of thermal shrinkage of the film. Consequently, the width of the film between both fixed ends becomes the same as the distance between both fixed ends, and thus the film without wrinkles can be produced.

In the present invention, step (D) may include a step (step (D-2)) of stretching the film in the TD direction. When step (D) further includes step (D-2), it is possible to control the coefficient of linear expansion ratio and the tear propagation resistance ratio of the film.

Specifically, in order to stabilize the coefficient of linear expansion ratio and the tear propagation resistance ratio of the polyimide film across the entire width and to produce the polyimide film having low values of the coefficient of linear expansion ratio and the tear propagation resistance ratio, the production method including step (D-2) may be employed.

In step (D-2) in the present invention, the film is stretched in the TD direction in the heating furnace after step (D-1). In step (D-1), the film is transferred while being fixed so that no tension is applied in the film width direction (TD direction). In this step, when the film is heated in the heating furnace, the film is shrunk to some extent. After the sag of the film is removed by shrinking, the film is stretched in the TD direction. The amount of stretching preferably satisfies the following formula: 40.0≧(W−Z)/Z×100>0.00  (12) wherein Z represents the width (for example, 41 in FIG. 5) between both fixed ends in the TD direction before stretching, and W represents the width (for example, 42 in FIG. 5) between both fixed ends after the film is stretched in the TD direction in the furnace. When the ratio (W−Z)/Z×100 (may be referred to as “TD expansion ratio” for convenience) exceeds the above range, it may be difficult to control the coefficient of linear expansion ratio and the tear propagation resistance ratio of the film to low values in a uniform level across the entire width. The ratio is more preferably 30.0≧(W−Z)/Z×100>0.00, and particularly preferably 20.0≧(W−Z)/Z×100>0.00. In step (D-2), the film may be stretched in the TD direction while the width of the film between the gripping positions is gradually increased. If necessary, shrinking may be again performed after step (D-2), and the width of the film may be further increased. Preferably, the TD shrinkage ratio and the TD expansion ratio are appropriately selected.

In order to control the coefficient of linear expansion ratio and the tear propagation resistance ratio of the film to desired ranges, the relationship between the TD shrinkage ratio and the TD expansion ratio preferably satisfies the following formula: 10.0≧TD shrinkage ratio−TD expansion ratio≧−10.0  (13)

More preferably, 8.0≧TD shrinkage ratio−TD expansion ratio≧−8.0, and particularly preferably 5.0≧TD shrinkage ratio−TD expansion ratio≧−5.0.

The temperature in step (D-2) is preferably in the range of 300° C. to 500° C. and particularly preferably in the range of 350° C. to 480° C. because the elastic modulus of the polyimide film is decreased, thereby easily stretching the film. At the above temperature, the film may be overstretched because of softening of the film. In such a case, the temperature is preferably appropriately set outside the above range.

Furthermore, in step (D-2), the coefficient of linear expansion ratio and the tear propagation resistance ratio of the film can be controlled to low values by adjusting the TD expansion ratio. That is, the coefficient of linear expansion ratio and the tear propagation resistance ratio of the film can be freely controlled by stretching the film in step (D-2).

In the present invention, shrinkage in step (D-1), stretching in step (D-2), film tension in the MD direction during transfer, the weight of the residual component of the gel film, and the heating temperature may be appropriately adjusted to produce the film having desired coefficient of linear expansion ratio and the tear propagation resistance ratio. The heating temperature and the heating time of the film are completely different depending on imidization method including chemical imidization or thermal imidization. However, even in thermal imidization, the intended film can be produced by controlling conditions according to the method of the present invention.

The heating furnace that is preferably used in the present invention includes an air-heating furnace in which hot air at 60° C. or higher is blasted on the entire film from the lower or upper surface or both surfaces of the film, and a far-infrared furnace including a far-infrared generator for baking the film by irradiating the film with far-infrared rays. In the heating step, the film is preferably baked by increasing the temperature stepwise. For this purpose, a heating furnace for increasing the temperature stepwise is preferably used in which a plurality of air-heating furnaces, far-infrared furnaces, or air-heating furnaces and far-infrared furnaces are connected to perform baking.

In the process of producing the polyimide film of the present invention, when the gel film is gripped and transferred in the furnace in the baking step, the first heating temperature is preferably 300° C. or lower, more preferably in the range of 60° C. to 250° C., and particularly preferably in the range of 100° C. to 200° C. in order to control the difference between the maximum A_(MAX) and the minimum A_(MIN) of the coefficient of linear expansion ratio A to a low value or control the difference between the maximum and the minimum of the tear propagation resistance ratio to a low value across the entire width of the film. Specifically, the film is preferably transferred through two or more heating furnaces and the temperature of a first heating furnace (32 in FIG. 5) is preferably controlled to 300° C. or lower. When the furnaces are applied to the production of another polyimide film, the heating temperature is preferably determined in consideration of the type of the polyimide film and the evaporation temperature of the solvent used. In particular, preferably, the boiling point of the solvent contained in the gel film is measured, and the heating temperature is controlled to a temperature not more than 100° C. higher than the boiling point of the solvent.

In the production of the polyimide film, when the first heating temperature in the furnace in which the gel film is transferred is higher than 300° C., bowing phenomenon (a phenomenon in which a strong molecular orientation occurs at the ends of the film because the central portion of the film is transferred in the heating furnace earlier than the ends thereof because of the influence of shrinkage of the film) may occur, resulting in a difficulty of controlling the orientation angles at the ends of the film. Furthermore, it may be difficult to control the difference between the maximum A_(MAX) and the minimum A_(MIN) of the coefficient of linear expansion ratio A and the difference between the maximum and the minimum of the tear propagation resistance ratio to low values. In the baking of the polyimide film, the temperature of a second furnace 33 shown in FIG. 5 is preferably set in the range of 50° C. to 300° C. higher than the temperature of the first furnace 32 shown in FIG. 5 and particularly preferably in the range of 60° C. to 250° C. higher than the temperature of the first furnace 32 shown in FIG. 5 in view of control of the molecular orientation angle of the polyimide film. In the subsequent furnaces, baking is preferably performed at temperatures that are generally used in the production of polyimide films. However, when the temperature of the first furnace (32 in FIG. 5) is 60° C. or lower, the temperature of the next furnace (33 in FIG. 5) is preferably set in the range of 100° C. to 250° C. When the temperature of the first furnace (32 in FIG. 5) is 60° C. or lower, the temperature of the second furnace is preferably set to the above value in order to control the difference between the maximum A_(MAX) and the minimum A_(MIN) of the coefficient of linear expansion ratio A to a low value or control the difference between the maximum and the minimum of the tear propagation resistance ratio to a low value across the entire width of the film. In the baking step, although the temperatures of the first furnace and the next furnace are set as described above, the other temperatures are preferably set to baking temperatures that are generally used in the production of polyimide films. In an example of a method, a polyimide film can be baked by increasing the temperature stepwise to 600° C. at the maximum, and then gradually cooled to room temperature. When the maximum baking temperature is low, imidization may not be completely performed. Therefore, sufficient baking is necessary.

When the gel film is transferred to the furnace, the tension applied to the gel film in the MD direction is calculated in terms of tension (load) applied per meter of the film. The tension is preferably in the range of 1 to 20 kg/m, more preferably in the range of 1 to 15 kg/m, and particularly preferably in the range of 1 to 10 kg/m. When the tension is less than 1 kg/m, it is difficult to stably transfer the film, and thus stable film production tends to become difficult while the film is gripped. When the tension applied to the film is more than 20 kg/m, in particular, it is difficult to control the orientation angles at the ends of the film, and in addition, the coefficient of linear expansion ratios and the tear propagation resistance ratios at the ends of the film are higher than those in the central portion. Consequently, it tends to become difficult to control the coefficient of linear expansion ratio and the tear propagation resistance ratio to uniform values across the entire width. Examples of a tension generator for applying tension to the gel film transferred to the furnace include a load roll for applying a load to the gel film, a roll for changing a load by adjusting the rotational speed of the roll, a nip roll for controlling tension by nipping the gel film between two rolls. The tension applied to the gel film can be adjusted by a method using the above tension generator.

Preferably, the tension applied to the film is appropriately adjusted in the above range depending on the thickness of the polyimide film. In view of formability of the polyimide film, the thickness of the film is preferably in the range of 1 to 200 μm and particularly preferably in the range of 1 to 100 μm. When the thickness of the film is more than 200 μm, the shrinkage stress generated in the film may be increased, and thus it may be impossible to control the coefficient of linear expansion ratio and the tear propagation resistance ratio of the film, and the orientation angle thereof.

The polyimide film produced by the production method of the present invention may be subjected to any desired process such as heat treatment, forming, surface treatment, lamination, coating, printing, embossing, or etching according to need.

(III) Use of the Invention

The polyimide film produced by the above-described production method of the present invention can be used for any application. Examples of the application include applications to electric and electronic device substrates such as base films for FPCs, TAB tapes, and high-density recording media; applications to solar cell substrates; applications to magnetic recording media; and applications to electric insulation. In particular, the polyimide film can be preferably used as the base film for FPC. As described in section (I), in the polyimide film of the present invention, in particular, the rate of dimensional change across the entire width can be small, and the dimensional stability across the entire width can be satisfactory before and after etching in the production process of FPC. As a result, the polyimide film can be suitably used as the base film of FPC for high-density mounting.

The polyimide film of the present invention may be a single layer film composed of only the polyimide film, but the polyimide film may be also a laminate including another layer. Specifically, for example, another polymer layer may be applied or laminated on at least one surface of the polyimide film. Examples of the other resin layer to be laminated include, but are not particularly limited to, layers of thermoplastic polyimides, polyesters, polyolefins, polyamides, polyvinylidene chloride, acrylic polymers, and fluorocarbon polymers. These polymer layers may be laminated directly on the polyimide film or with an adhesive layer therebetween.

After a gel film is formed, for example, the laminate can be preferably produced by the following method: (1) a method of dipping the gel film in a solution (referred to as “another resin solution”) prepared by dissolving another resin, and then drying the film in a tenter furnace by heating to produce a laminated film, (2) a method of applying another resin solution on a surface of the gel film using a coater, and then drying the film by heating to produce a laminated film, or (3) a method of spray-coating another resin solution on the gel film using a spray device, and then drying the film by heating to produce a laminated film. Alternatively, another resin solution may be again applied on a surface of the formed polyimide film, and then dried by heating to produce a laminated film. In this case, any one of the lamination methods (1) to (3) is preferably used as the application method. The laminate (or laminated film) in the present invention may be any laminate that includes at least one layer of the other resin.

Furthermore, examples of a method that is preferably used for producing a metal-clad laminate (laminate) in which a metal is laminated with the polyimide film include (1) a method of laminating an adhesive layer on at least one surface of the polyimide film, and then performing thermocompression bonding of a metal on the adhesive layer (preferably using, for example, a pressing method, a double belt method, or a hot roll method); (2) a method of laminating (vacuum laminating) a metal directly on at least one surface of the polyimide film in a vacuum apparatus; (3) a method of thickly laminating a metal layer on the metal-clad laminate, which is produced by the vacuum lamination in method (2), by metal plating or electroless plating; (4) a method of thinly laminating a metal by electroless plating; and (5) a method of thickly laminating a metal layer on the metal layer, which is thinly laminated by electroless plating in method (4), by metal plating or electroless plating.

The metal-clad laminate produced as described above is subjected to a wiring formation process for the metal layer (by, for example, a method of forming an etching mask on the surface, and then etching the metal layer) to form metal wiring on the base film including at least the polyimide film.

In method (1) among methods of (1) to (5) that can be preferably used in the present invention, examples of the adhesive that can be suitably used include, but are not particularly limited to, thermoplastic polyimide resin adhesives (adhesives containing at least a polyimide resin), acrylic adhesives (adhesives containing at least an acrylic resin), and epoxy adhesives (adhesives containing at least an epoxy resin). In production method (1), a metal foil of copper, aluminum, gold, silver, nickel, chromium, or an alloy thereof having a thickness of at least 0.1 μm is used as the metal. Furthermore, in the laminate having the adhesive layer, which is produced by production method (1), a protective material may be laminated for protecting the adhesive layer.

Examples of a method that is preferably used for laminating in the vacuum apparatus in production method (2) include a heating evaporation method in which a metal is laminated by heating evaporation in a heating furnace, an electron beam method in which a metal is laminated by heating evaporation using an electron beam, and a sputtering method in which a metal is laminated by evaporation using plasma. The metal used in this method is not particularly limited, but copper, gold, silver, manganese, nickel, chromium, titanium, tin, cobalt, indium, molybdenum, or the like is preferably used. These metals may be used alone. Alternatively, a plurality of the types of the metals may be simultaneously evaporated to form a metal alloy on a surface of the polyimide film. Examples of such a method include a method of simultaneously laminating nickel and chromium to form a nickel/chromium alloy, and a method of simultaneously evaporating indium and tin in the presence oxygen to form an indium tin oxide (ITO) film.

Furthermore, in a method of electroless plating used in production method (4), a catalyst metal for electroless plating is laminated on a surface of the polyimide film, and the polyimide film is then immersed in a metal-containing bath for electroless plating to laminate the metal. The electroless plating method is not limited to the above method, and a method of laminating a metal by a known electroless plating technique can be preferably used.

In a method of electroplating in production methods (3) and (5), for example, the metal-clad polyimide film produced by production method (2) or (4) is immersed in a solution prepared by dissolving a metal to be plated, and a current is then supplied using the metal to be electroplated as a counter electrode to perform plating. The electroplating method is not limited to the above method, and a method of laminating a metal by a known electroplating technique can be preferably used.

In a method of thickly laminating the metal by electroless plating, for example, a film prepared by applying a catalyst for electroless plating on a surface of the metal-clad polyimide film produced by production method (2) or (4) is immersed in an electroless plating bath prepared by dissolving a desired metal to laminate the metal. The electroless plating method is not limited to the above method, and a method of laminating a metal by a known electroless plating technique can be preferably used.

A protective material for protecting the metal layer may be laminated on the metal-clad polyimide film produced by production methods (1) to (5).

As described above, the laminate of the present invention is not particularly limited as long as it includes the polyimide film of the present invention. Furthermore, although representative examples of the method of producing the metal laminate have been descried in detail above, the present invention in not limited thereto. Accordingly, the present invention also includes a method of producing a metal laminate (for example, a metal-clad laminate for FPC, TAB, high-density recording media, magnetic recording media, or electrical and electronic devices) produced by using the polyimide film as a base film and the above flexible metal-clad laminate. The method of producing the FPC or the flexible metal-clad laminate is not limited to the above-described methods. Various known methods that can be used by those skilled in the art may be used to laminate the metal layer.

EXAMPLES

The present invention will now be specifically described on the basis of examples, but is not limited by these examples. Persons skilled in the art can make various changes, adjustments, and modifications without departing from the scope of the present invention. The coefficient of linear expansion ratio, the tear propagation resistance ratio, and the molecular orientation angle of the polyimide film of the present invention, the rate of dimensional change of the flexible metal-clad laminate, and the elastic modulus were measured and evaluated as described below.

[Coefficient of Linear Expansion Ratio]

As shown in FIG. 2, strip measurement samples 3 and 4 of a polyimide film 2 were respectively cut out along the direction of the molecular orientation axis (D_(AL) in the figure) and the perpendicular direction (D_(VE)). The coefficient of linear expansion a in the direction of the molecular orientation axis was measured using the measurement sample 3 cut out along the direction of the molecular orientation axis. The coefficient of linear expansion b in the perpendicular direction was measured using the measurement sample 4 cut out along the perpendicular direction. A thermomechanical analyzer (manufactured by Seiko Instruments Inc., trade name: TMA120C) was used as the measurement device. The samples were heated in the temperature range of 10° C. to 400° C. under a nitrogen stream at a heating rate of 10° C./min. The measured values in the temperature range of 100° C. to 200° C. were averaged.

The coefficient of linear expansion ratio A of the polyimide film 2 was calculated from the coefficients of linear expansion a and b in the respective directions using equation (1). Furthermore, the difference between the maximum A_(MAX) and the minimum A_(MIN) (difference A_(DIF) in coefficient of linear expansion ratio=A_(MAX)−A_(MIN)) was calculated from the calculated coefficient of linear expansion ratio A.

[Molecular Orientation Angle]

The molecular orientation angles at both end portions and at the central portion of the polyimide film were measured with a molecular orientation analyzer (manufactured by Oji Scientific Instruments, trade name: MOA 2012). Specifically, as shown in FIG. 3, it was confirmed whether the molecular orientation angle was within ±20° on the basis of the reference (0°) of the MD direction shown by 10 in FIG. 3, when the MD direction (14 in FIG. 3, i.e., the transfer direction of the polyimide film) and the direction perpendicular to the MD direction (15 in FIG. 3, i.e., the TD direction) were determined. Furthermore, the difference in molecular orientation angle was calculated from the difference between the maximum and the minimum of the calculated molecular orientation angles.

[Rate of Dimensional Change]

As shown in FIG. 7, in a rolled flexible metal-clad laminate 50, the flexible metal-clad laminate 50 was pulled out and measurement samples 51, 52, and 53 each having desired dimensions were prepared from both end portions and the central portion of the laminate along the width direction. As shown in FIG. 8, the rates of dimensional change of the measurement samples 51 to 53 were measured (1) in the measurement direction along the MD direction (an arrow D₁ in the figure) shown by double-ended arrows D_(MD) in the figure, (2) in the direction along the TD direction (the direction perpendicular to the MD direction) shown by double-ended arrows D_(TD) in the figure, (3) in the direction (R direction) tilted by 45° rightward from the MD direction shown by a double-ended arrow D_(R) in the figure, and (4) in the direction (L direction) tilted by 45° (−45° when the rightward tilt is defined as being on the plus side) leftward from the MD direction shown by a double-ended arrow D_(L) in the figure.

The dimensional change was measured according to the method specified in Japanese Industrial Standards (JIS) C6481. Specifically, holes 60 (see FIG. 8) for measurement were formed at four corners of each of the measurement samples 51 to 53. The distances between the holes 60, that is, the dimensions in the D_(MD), D_(TD), D_(R), and D_(L) directions were measured. Next, the measurement samples 51 to 53 were etched to remove a metal foil, and were then left to stand in a thermostatic chamber at 20° C. and 60% R.H. for 24 hours.

Subsequently, the distances between the holes 60 were measured as before the etching. The rate of dimensional change before and after the etching was calculated using the following equation (14): Rate of dimensional change (%)={(M ₂ −M ₁)/M ₁}×100  (14) wherein M₁ is the measured distance between the holes before the removal of the metal foil and M₂ is the measured distance between the holes after the removal of the metal foil. [Measurement of Tear Propagation Resistance and Tear Propagation Resistance Ratio]

Samples for measuring the molecular orientation angle described below were prepared from the both end portions and the central portion, and the molecular orientation angle was measured. The molecular orientation axis was determined from the measurement result of the molecular orientation angle. As shown in FIG. 4, from the above samples, test pieces (10 mm×20 mm) were respectively cut out in a direction of the molecular orientation axis and in a direction perpendicular to the molecular orientation axis. The measurement was performed in accordance with ASTM D1938 using the above test pieces.

[Measurement of Elastic Modulus]

Test pieces (15 mm×200 mm) were respectively cut out in the MD direction and the TD direction using the part 5 shown in FIG. 1. The measurement was performed in accordance with ASTM-D882 with a tensile testing machine manufactured by Shimadzu Corporation (Autograph S-100-C) using the test pieces.

Example 1 Production of Polyimide Film

To an organic solvent N,N-dimethylformamide (DMF) for polymerization, 50 mole percent of 4,4-diaminodiphenyl ether (ODA), 50 mole percent of p-phenylenediamine (p-PDA), 50 mole percent of p-phenylenebis(trimellitic acid monoester anhydride) (TMHQ), and 50 mole percent of pyromellitic dianhydride (PMDA) were added and polymerized under stirring to synthesize a polyamic acid solution. The synthesis was performed so that the solid content of the resulting polyamic acid solution was 15 weight percent.

To the polyamic acid solution, 2.0 equivalents of acetic anhydride and 1.0 equivalent of isoquinoline relative to the equivalent of the amic acid were added, and the solution was cast onto an endless belt so that the layer of the solution had a thickness of 20 μm at a width of 1,100 mm. The solution was dried with hot air in the temperature range of 100° C. to 150° C. for two minutes to prepare a self-supporting gel film (polyimide precursor film). The residual component ratio of the gel film was 54 weight percent. The gel film was peeled off from the endless belt, and both ends of the gel film in the width direction were fixed without a sag on pin seats that continuously transfer the film. The distance between the pin seats was 1,000 mm.

The gel film was baked stepwise in a first heating furnace (177° C.), a second heating furnace (300° C.), a third heating furnace (450° C.), and a fourth heating furnace (515° C.) to advance imidization, thereby obtaining a polyimide film. In this process, the gel film was transferred to the tenter furnace and in the tenter furnace while being shrunk and/or expanded in the TD direction so as to adjust the TD shrinkage ratio to 4.40 and the TD expansion ratio to 2.20. The step of decreasing the distance between both fixed ends so that substantially no tension was applied to the film in the TD direction was completed before the film was introduced into the furnace. The step of increasing the distance between both fixed ends was performed in the third heating furnace.

The conditions for producing the polyimide film are shown in Table 1.

The molecular orientation angles, the difference in molecular orientation angle, the coefficients of linear expansion, the coefficient of linear expansion ratios A, the difference A_(DIF) in coefficient of linear expansion ratio, the tear propagation resistances, the tear propagation resistance ratios, the difference between the maximum and the minimum of the tear propagation resistance ratio, and the angles of the molecular orientation axis of the prepared polyimide film were measured. The results are shown in Tables 2 and 3. In Table 2, rows corresponding to an end portion, the central portion, and another end portion are provided for each example (or comparative example) to show the results of the measurement samples 51, 52, and 53. As is apparent from the results shown in Tables 2 and 3, in the polyimide film, the coefficient of linear expansion ratio A was controlled in the range of 1.01 to 3.00, the difference A_(DIF) in coefficient of linear expansion ratio was controlled to 0.30 or less, the tear propagation resistance ratio was controlled in the range of 1.01 to 1.20, the difference between the maximum and the minimum of the tear propagation resistance ratio was controlled to 0.10 or less, and the angle of the molecular orientation axis was controlled to 0±20° across the entire width of the film. Furthermore, the average coefficient of linear expansion C_(LE) was calculated using the following equation (2). In equation (2), as shown in FIG. 2, the coefficient of linear expansion in a direction of the molecular orientation axis (D_(AL) in the figure) at an end is defined as C1 _(AL), the coefficient of linear expansion in the perpendicular direction (D_(VE)) at the end is defined as C1 _(VE), the coefficient of linear expansion in the direction of the molecular orientation axis (D_(AL) in the figure) at the center is defined as C0 _(AL), the coefficient of linear expansion in the perpendicular direction (D_(VE)) at the center is defined as C0 _(VE), the coefficient of linear expansion in the direction of the molecular orientation axis (D_(AL) in the figure) at another other end is defined as C2 _(AL), and the coefficient of linear expansion in the perpendicular direction (D_(VE)) at the other end is defined as C2 _(VE). C _(LE)={(C1_(AL) +C1_(VE))/2+(C0_(AL) +C0_(VE))/2+(C2_(AL) +C2_(VE))/2}/3  (2) The average coefficient of linear expansion C_(LE) calculated from the equation (2) was 12.8 ppm/° C.

The polyimide film had an elastic modulus of 6.1 GPa.

<Synthesis of Thermoplastic Polyimide Precursor Used as Adhesive Layer>

To an organic solvent DMF for polymerization, 100 mole percent of bis[4-(4-aminophenoxy)phenyl]sulfone (BAPS), 90 mole percent of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), and 10 mole percent of 3,3′,4,4′-ethylene glycol benzoate tetracarboxylic dianhydride (TMEG) were added and polymerized under stirring to synthesize a polyamic acid solution serving as the precursor of a thermoplastic polyimide. The synthesis was performed so that the solid content of the resulting polyamic acid solution was 20 weight percent.

<Production of Flexible Metal-Clad Laminate>

The polyimide film was preliminarily subjected to a surface plasma treatment by performing plasma discharge at an output of 280 W/m² in a mixed gas stream of Ar:He:N₂=8:2:0.2 (volume ratio) on the surface of the polyimide film. The above-described thermoplastic polyimide precursor was diluted with DMF to a solid content of 10 weight percent. Subsequently, the diluted thermoplastic polyimide precursor was applied on both surfaces of the polyimide film across the entire width so that the final thickness of the thermoplastic polyimide layer at one surface was 4 μm, and the polyimide film was then heated at 140° C. for one minute. The polyimide film was then passed through a far-infrared heating furnace having an atmospheric temperature of 390° C. for 20 seconds to conduct imidization. Thus, adhesive layers composed of the thermoplastic polyimide precursor were formed. Thereby, a three-layer laminate in which adhesive layers were laminated on both surfaces of the polyimide film was prepared.

An 18-μm rolled copper foil (manufactured by Japan Energy Corporation, trade name: BHY-22B-T) was disposed on each surface of the three-layer laminate, and a protective material (manufactured by Kaneka Corporation, trade name: Apical 125NPI) was applied on the surface of each copper foil. The resulting laminate was thermally laminated at a lamination temperature of 380° C., a lamination pressure of 196 N/cm (20 kgf/cm), and a lamination rate of 1.5 m/min while a tension of 0.4 N/cm was applied to the polyimide film. The hot lamination was continuously performed to produce a flexible metal-clad laminate.

After the lamination, the protective material was removed from the flexible metal-clad laminate. The above-described samples for measuring the rate of dimensional change were prepared from the flexible metal-clad laminate, and the rates of dimensional change before and after etching of each measurement sample were measured by the above method. The etching was performed with a device as follows. A hydrochloric acid solution (concentration: 30% or higher) of ferric chloride manufactured by Harima Kagaku Kogyo K.K. was heated to 30° C. with a heater, and the heated solution was sprayed from above and below the film to expose each surface of the film to the solution. The time of contact between the ferric chloride solution and the metal-clad laminate was set to within 10 minutes. In the etching process, the time was adjusted according to the etching rate. After the etching, the film was washed with water and dried by blowing off the droplets with air. Thus, a film with the copper layers removed was produced.

The rates of dimensional change of the polyimide film before and after etching were measured. The results are shown in Table 4. In Table 4, as in Tables 2 and 3, rows corresponding to an end portion, the central portion, and another end portion are provided for each example (or comparative example) to show the results of the measurement samples 51, 52, and 53.

As is apparent from the results, the polyimide film of the present invention had small rates of dimensional change and was suitable for a base film.

Example 2

A polyimide film was prepared as in Example 1 except that the TD shrinkage ratio was 4.40 and the TD expansion ratio was 4.40 as shown in Table 1. A flexible metal-clad laminate was produced as in Example 1 using the prepared polyimide film.

The physical property values of the polyimide film and the flexible metal-clad laminate were evaluated by the same methods as those in Example 1. The results are shown in Tables 2 to 4. As is apparent from the results, the use of the polyimide film of the present invention reduced the rates of dimensional change after etching and stabilized the rates of dimensional change across the entire width.

The polyimide film had an elastic modulus of 5.9 GPa.

Example 3

A polyimide film was prepared as in Example 1 except that the TD shrinkage ratio was 3.90 and the TD expansion ratio was 0.00, and that the initial temperature in the tenter furnace was 130° C., and the film was further baked stepwise at 250° C., 350° C., 450° C., and 515° C. to conduct imidization, as shown in Table 1. A flexible metal-clad laminate was produced as in Example 1 using the prepared polyimide film.

The physical property values of the polyimide film and the flexible metal-clad laminate were evaluated by the same methods as those in Example 1. The results are shown in Tables 2 to 4. As is apparent from the results, the use of the polyimide film of the present invention reduced the rates of dimensional change after etching and stabilized the rates of dimensional change across the entire width.

The polyimide film had an elastic modulus of 6.1 GPa.

Example 4

A polyimide film was prepared as in Example 1 except that the TD shrinkage ratio was 2.00 and the TD expansion ratio was 0.00, and that the initial temperature in the tenter furnace was 130° C., and the film was further baked stepwise at 250° C., 350° C., 450° C., and 515° C. to conduct imidization, as shown in Table 1. A flexible metal-clad laminate was produced as in Example 1 using the prepared polyimide film.

The physical property values of the polyimide film and the flexible metal-clad laminate were evaluated by the same methods as those in Example 1. The results are shown in Tables 2 to 4. As is apparent from the results, the use of the polyimide film of the present invention reduced the rates of dimensional change after etching and stabilized the rates of dimensional change across the entire width.

The polyimide film had an elastic modulus of 5.8 GPa.

Example 5

A polyimide film was prepared as in Example 1 except that the TD shrinkage ratio was 4.00 and the TD expansion ratio was 0.00, and that the initial temperature in the tenter furnace was 160° C., and the film was further baked stepwise at 300° C., 450° C., and 515° C. to conduct imidization, as shown in Table 1. A flexible metal-clad laminate was produced as in Example 1 using the prepared polyimide film.

The physical property values of the polyimide film and the flexible metal-clad laminate were evaluated by the same methods as those in Example 1. The results are shown in Tables 2 to 4. As is apparent from the results, the use of the polyimide film of the present invention reduced the rates of dimensional change after etching and stabilized the rates of dimensional change across the entire width.

The polyimide film had an elastic modulus of 6.0 GPa.

Example 6

A polyimide film was prepared as in Example 1 except that the TD shrinkage ratio was 3.00 and the TD expansion ratio was 0.00, and that the initial temperature in the tenter furnace was 170° C., and the film was further baked stepwise at 300° C., 450° C., and 515° C. to conduct imidization, as shown in Table 1. A flexible metal-clad laminate was produced as in Example 1 using the prepared polyimide film.

The physical property values of the polyimide film and the flexible metal-clad laminate were evaluated by the same methods as those in Example 1. The results are shown in Tables 2 to 4. As is apparent from the results, the use of the polyimide film of the present invention reduced the rates of dimensional change after etching and stabilized the rates of dimensional change across the entire width.

The polyimide film had an elastic modulus of 6.0 GPa.

Example 7

A polyimide film was prepared as in Example 1 except that the TD shrinkage ratio was 5.00 and the TD expansion ratio was 0.00, and that the initial temperature in the tenter furnace was 165° C., and the film was further baked stepwise at 300° C., 450° C., and 515° C. to conduct imidization, as shown in Table 1. A flexible metal-clad laminate was produced as in Example 1 using the prepared polyimide film.

The physical property values of the polyimide film and the flexible metal-clad laminate were evaluated by the same methods as those in Example 1. The results are shown in Tables 2 to 4. As is apparent from the results, the use of the polyimide film of the present invention reduced the rates of dimensional change after etching and stabilized the rates of dimensional change across the entire width.

The polyimide film had an elastic modulus of 6.1 GPa.

Comparative Example 1

A comparative polyimide film was prepared as in Example 1 except that the TD shrinkage ratio was 0.00 and the TD expansion ratio was 0.00 as shown in Table 1 (“Comparative Example 1” in Table 1). A comparative flexible metal-clad laminate was produced as in Example 1 using the prepared comparative polyimide film.

The physical property values of the comparative polyimide film and the comparative flexible metal-clad laminate were evaluated by the same methods as those in Example 1. The results are shown in Tables 2 to 4 (“Comparative Example 1” in Tables 2, 3, and 4). As is apparent from the results, the use of the comparative polyimide film increased the rates of dimensional change after etching, and particularly increased the amounts of dimensional change in the D_(R) and D_(L) directions. Thus, it was evident that a flexible metal-clad laminate having stable rates of dimensional change across the entire width could not be obtained.

The polyimide film had an elastic modulus of 6.1 GPa.

TABLE 1 Initial Stretch- tempera- ing TD Residual ture tempera- TD expan- component in furnace ture shrinkage sion ratio (%) (° C.) (° C.) ratio ratio Example 1 54 177 450 4.40 2.20 Example 2 54 177 450 4.40 4.40 Example 3 60 130 — 3.90 0.00 Example 4 60 130 — 2.00 0.00 Example 5 52 160 — 4.00 0.00 Example 6 71 170 — 3.00 0.00 Example 7 68 165 — 5.00 0.00 Comparative 54 177 — 0.00 0.00 Example 1

TABLE 2 Coefficient of linear Difference expansion in Direction Average Molecular molecular of coefficient Coefficient Difference A_(DIF) orientation orientation molecular Perpendicular of linear of linear in coefficient angle angle orientation direction expansion C_(LE) expansion of linear Position (degrees) (degree) ppm/° C. (ppm/° C.) ratio A expansion ratio Example 1 End 0 9 11.5 14.2 12.8 1.21 0.10 Center −3 12.0 13.6 1.13 End 6 11.3 14.1 1.22 Example 2 End −5 16 9.59 16.74 13.1 1.54 0.12 Center −1 10.3 15.9 1.43 End 11 10 15.8 1.45 Example 3 End −1 6 10.1 14.1 12.8 1.33 0.02 Center 0 11.2 15.3 1.31 End 5 10.9 15.1 1.32 Example 4 End −6 11 11.5 17.9 14.5 1.44 0.02 Center 1 11.4 18.1 1.45 End 5 10.9 17.3 1.45 Example 5 End −3 17 9.6 18.2 13.5 1.62 0.05 Center −6 8.6 17.3 1.67 End 11 9.1 18.1 1.66 Example 6 End −13 26 11.2 15.3 13.3 1.31 0.09 Center 5 11.5 15 1.26 End 13 11.9 14.8 1.22 Example 7 End −15 31 7.3 18 12.8 1.85 0.13 Center 10 7 18.4 1.90 End 16 8 17.9 1.76 Comparative End −45 90 8.66 17.2 12.6 1.66 0.49 Example 1 Center −6 10.59 12.92 1.20 End 45 8.67 17.73 1.69

TABLE 3 Tear propagation resistance Direction of Difference between the molecular Perpendicular Tear propagation maximum and the minimum orientation direction resistance ratio of tear propagation Position g/mm TD/MD resistance ratio Example 1 End 160 166 1.04 0.03 Center 149 153 1.03 End 155 163 1.05 Example 2 End 156 171 1.10 0.02 Center 142 154 1.08 End 160 172 1.07 Example 3 End 141 147 1.05 0.01 Center 123 128 1.04 End 135 141 1.04 Example 4 End 152 163 1.07 0.00 Center 146 157 1.08 End 156 168 1.08 Example 5 End 160 178 1.11 0.01 Center 155 175 1.13 End 163 183 1.12 Example 6 End 155 160 1.03 0.02 Center 148 155 1.05 End 156 162 1.04 Example 7 End 150 177 1.18 0.07 Center 140 175 1.25 End 154 182 1.18 Comparative End 131 147 1.12 0.12 Example 1 Center 120 122 1.01 End 135 153 1.13

TABLE 4 Rate of dimensional change after etching MD TD R L Position % Example 1 End −0.03 −0.04 −0.01 −0.06 Center −0.03 −0.03 −0.03 −0.03 End −0.03 −0.04 −0.01 −0.06 Example 2 End −0.03 −0.05 −0.04 −0.04 Center −0.02 −0.06 −0.05 −0.04 End −0.04 −0.04 −0.06 −0.02 Example 3 End −0.03 −0.05 −0.04 −0.03 Center −0.04 −0.04 −0.05 −0.03 End −0.03 −0.05 −0.02 −0.02 Example 4 End −0.04 −0.02 −0.03 −0.04 Center −0.04 0.00 −0.02 −0.02 End −0.03 −0.02 −0.02 −0.03 Example 5 End 0.02 −0.05 −0.01 0.00 Center 0.03 −0.06 −0.03 0.01 End 0.04 −0.05 −0.02 0.02 Example 6 End −0.03 −0.04 −0.04 −0.03 Center −0.04 −0.05 −0.03 −0.05 End −0.03 −0.05 −0.02 −0.02 Comparative End −0.03 0.02 0.09 −0.10 Example 1 Center −0.07 −0.01 0.00 0.02 End −0.11 0.03 −0.10 0.07

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a method of obtaining samples for determining the molecular orientation angle and the molecular orientation axis.

FIG. 2 shows a method of sampling films for measuring the coefficient of linear expansion.

FIG. 3 is a view illustrating the molecular orientation axis and the molecular orientation angle of a film.

FIG. 4 shows a method of sampling films for measuring the tear propagation resistance.

FIG. 5 includes schematic views showing a production process and a transfer process of a film.

FIG. 6 is a schematic view showing a state in which a film is gripped.

FIG. 7 is a schematic view showing positions of samples prepared from an FPC.

FIG. 8 is a schematic view showing positions for measuring dimensional changes of a sample for measuring the rates of dimensional change.

Reference Numerals 1 40 mm 2 polyimide film 3 measurement sample (direction of molecular orientation axis) 4 measurement sample (perpendicular direction) 5 measurement part of elastic modulus 10 MD direction (mechanical feeding direction of film) 11 positive (plus) molecular orientation angle 12 negative (minus) molecular orientation angle 13 TD direction (direction perpendicular to mechanical feeding direction of film) 14 MD direction 15 TD direction 20 molecular orientation axis 21 sample film (direction of molecular orientation axis) 22 sample film (direction perpendicular to molecular orientation axis) 23 direction perpendicular to molecular orientation axis 24 10 mm 25 10 mm 26 20 mm 31 film transfer apparatus 32 first heating furnace 33 second heating furnace 34 third heating furnace 35 fourth heating furnace 36 fifth heating furnace 37 film-gripping start position 38 width X between gripping devices of film 39 film width Y in the TD direction of gel film gripped by gripping devices 40 film-transferring direction 41 width Z between both fixed ends in the TD direction before stretching 42 width W between both fixed ends after film is stretched in the TD direction in furnace 43 unit (die) for casting or applying organic solvent solution 44 base for applying organic solvent solution 45 unit for applying tension to gel film 46 position from which gel film is peeled off 47 step of taking up to takeup unit (takeup unit of polyimide film) 50 flexible metal-clad laminate 51 measurement sample (an end portion) 52 measurement sample (central portion) 53 measurement sample (another end portion) 60 hole for measurement 

1. A polyimide film produced by a continuous process, wherein when a coefficient of linear expansion a in a direction of the molecular orientation axis and a coefficient of linear expansion b in a direction perpendicular to the molecular orientation axis are measured in the temperature range of 100° C. to 200° C., a coefficient of linear expansion ratio A represented by equation (1): A=1+{(b−a)/(b+a)}×2  (1) is in the range of 1.13 to 3.00 across the entire width.
 2. The polyimide film according to claim 1, wherein the difference between the maximum A_(MAX) and the minimum A_(MIN) of the coefficient of linear expansion ratio A is 0.30 or less.
 3. The polyimide film according to claim 1, wherein the difference between the maximum and the minimum of the molecular orientation angle of the polyimide film is 40° or less across the entire width.
 4. The polyimide film according to claim 1, wherein when the transferring direction (MD direction) during the continuous production is defined as 0°, the molecular orientation angle is within 0±20°.
 5. The polyimide film according to claim 1, wherein the average coefficient of linear expansion in the temperature range of 100° C. to 200° C. is in the range of 5.0 to 25.0 ppm/° C.
 6. A laminate comprising the polyimide film according to claim
 1. 7. The laminate according to claim 6, further comprising at least a metal layer.
 8. A flexible printed circuit board comprising the polyimide film according to claim 1 as a base film.
 9. A polyimide film produced by a continuous process, wherein when a tear propagation resistance c in a direction of the molecular orientation axis and a tear propagation resistance d in a direction perpendicular to the molecular orientation axis are measured, the tear propagation resistance ratio d/c is in the range of 1.01 to 1.20 and the difference between the maximum and the minimum of the tear propagation resistance ratio d/c is 0.10 or less across the entire width.
 10. The polyimide film according to claim 9, wherein the difference between the maximum and the minimum of the molecular orientation angle of the polyimide film is 40° or less across the entire width.
 11. The polyimide film according to claim 9, wherein when the transferring direction (MD direction) during the continuous production is defined as 0°, the molecular orientation angle of the polyimide film is within 0±20° across the entire width.
 12. A laminate comprising the polyimide film according to claim
 9. 13. The laminate according to claim 12, further comprising at least a metal layer.
 14. A flexible printed circuit board comprising the polyimide film according to claim 9 as a base film.
 15. The polyimide film according to claim 2, wherein the difference between the maximum and the minimum of the molecular orientation angle of the polyimide film is 40° or less across the entire width.
 16. The polyimide film according to claim 2, wherein when the transferring direction (MD direction) during the continuous production is defined as 0°, the molecular orientation angle is within 0±20°.
 17. The polyimide film according to claim 3, wherein when the transferring direction (MD direction) during the continuous production is defined as 0°, the molecular orientation angle is within 0±20°.
 18. A laminate comprising the polyimide film according to claim
 4. 19. A laminate comprising the polyimide film according to claim
 5. 20. The polyimide film according to claim 10, wherein when the transferring direction (MD direction) during the continuous production is defined as 0°, the molecular orientation angle of the polyimide film is within 0±20° across the entire width. 